Natural killer (NK) cells are cytotoxic lymphocytes that constitute a major component of the innate immune system. NK cells do not express T-cell antigen receptors (TCR), CD3 or surface immunoglobulins (Ig) B cell receptor. NK cells generally express the surface markers CD16 (FcγRIII) and CD56 in humans, but a subclass of human NK cells is CD16−. NK cells are cytotoxic; small granules in their cytoplasm contain special proteins such as perforin and proteases known as granzymes. Upon release in close proximity to a cell targeted for killing, perforin forms pores in the cell membrane of the target cell through which the granzymes and associated molecules can enter, inducing apoptosis. One granzyme, granzyme B (also known as granzyme 2 and cytotoxic T-lymphocyte-associated serine esterase 1), is a serine protease crucial for rapid induction of target cell apoptosis in the cell-mediated immune response.
NK cells are activated in response to interferons or macrophage-derived cytokines. Activated NK cells are referred to as lymphokine activated killer (LAK) cells. NK cells possess two types of surface receptors, labeled “activating receptors” and “inhibitory receptors,” that control the cells' cytotoxic activity.
Among other activities, NK cells play a role in the host rejection of tumors. Because cancer cells have reduced or no class I MHC expression, they can become targets of NK cells. Accumulating clinical data suggest that haploidentical transplantation of human NK cells isolated from peripheral blood monomuclear cells (PBMC) or bone marrow mediate potent anti-leukemia effects without incurring detectable graft versus host disease (GVHD). See Ruggeri et al., Science 295:2097-2100 (2002)). Natural killer cells can become activated by cells lacking, or displaying reduced levels of, major histocompatibility complex (MHC) proteins. Additionally, the activating receptors expressed on NK cells are known to mediate detection of “stressed” or transformed cells with express ligands to activating receptors and therefore trigger the NK cell activation. For instance, NCR1 (NKp46) binds viral hemagglutinins. NKG2D ligands include CMV UL 16-binding protein 1 (ULB1), ULB2, ULB3 and MHC-class-I-polypeptide-related sequence A (MICA) and MICB proteins. NK protein 2B4 binds CD48, and DNAM-1 binds Poliovirus receptor (PVR) and Nectin-2, both are consistently detected in acute myeloid leukemia (AML). See Penda et al., Blood 105: 2066-2073 (2004). Moreover, lysis of AML has been described to be mainly natural cytotoxicity receptor (NCR) dependent. See Fauriat et al., Blood 109: 323-330 (2007). Activated and expanded NK cells and LAK cells from peripheral blood have been used in both ex vivo therapy and in vivo treatment of patients having advanced cancer, with some success against bone marrow related diseases, such as leukemia; breast cancer; and certain types of lymphoma. LAK cell treatment requires that the patient first receive IL-2, followed by leukopheresis and then an ex vivo incubation and culture of the harvested autologous blood cells in the presence of IL-2 for a few days. The LAK cells must be reinfused along with relatively high doses of IL-2 to complete the therapy. This purging treatment is expensive and can cause serious side effects. These include fluid retention, pulmonary edema, drop in blood pressure, and high fever.
In spite of the advantageous properties of NK cells in killing tumor cells and virus-infected cells, they remain difficult to work with and to apply in immunotherapy, primarily due to the difficulty in maintaining their tumor-targeting and tumoricidal capabilities during culture and expansion. Thus, there is a need in the art to develop an efficient method to produce and expand natural killer cells that retain tumoricidal functions.